Maybe, and maybe not. It must be pointed out that the technology we have today is extremely limited in regards to space travel. Based on what we know, yes, it would take years for a spaceship to traverse space. Of course, with the time dialation effect while traveling (read up on Einstein), there may not be much need for reproduction, a person could travel for millenia at close to light speed and only age maybe 30 or so years. And the speeds that such a spacecraft could travel at, even if not light speed, would give us minimal warning time. Maybe hours at most.
Myself, I am not convinced that the last thing has been said about space travel. We are clearly infants in this regard. U.F.O.‘s demonstrate a technology that, if they exist, and Blue Book indicates they do exist, is cleary way superior to our own, maybe in every way. Much of it we could not possibly comprehend with our limited knowledge. Instantaneous travel may be possible, utilizing principles we have no current knowledge of.
If they do come over our capitals and cities with the intent of conquest, there would be nothing we could do to stop them. Case closed.
The fact that they have not indicates that they will not. And that is the only margin we have at this moment.